The use of fiber optic sensors to determine the presence or magnitude of an external parameter is generally known in the art. One type of sensor is configured with a fiber optic waveguide that is interconnected between a light emitting source and a receiver. The waveguide comprises one or more optical fibers, each fiber including a glass core portion surrounded by a cladding (usually glass) with a relatively lower index of refraction. The cladding causes total internal reflection of light traveling in the core, thus trapping the light emitted from the source in the core of the fiber and allowing it to propagate with low loss over long distance.
A protective coating is applied to the glass fiber to protect it from chemical or mechanical damage that could reduce the transmissive qualities of the fiber. These protective coatings are generally composed of an epoxy acrylate and have a thickness betwen 0.5 and 1.5 times the fiber diameter so that they typically account for as much as 94% of the cross-sectional area of the coated fiber.
The coated waveguide is curved or coiled and typically mounted on a likewise curved support member. The curved support member is responsive to a mechanical element that is subject to movement caused by changes in the state of the external parameter. This movement, which is transmitted to the fiber optic waveguide, changes the curvature of the waveguide. The change in curvature results in a change in the intensity of the light transmitted by the light source through the fiber optic waveguide. That is, as the radius of curvature in a fiber optic waveguide decreases, the intensity of the transmitted light is increasingly attenuated. This type of attenuation, known as "bending loss" is generally attributed to radiation of the light out of the fiber at bent portions of the fiber optic waveguide.
The bending loss, determined by a straightforward comparison of the received and source signal, can be readily related to the position or magnitude of movement of the mechanical element, or to the force that caused that movement.
A typical example of a fiber optic sensor of the type just described is disclosed in the patent issued to Couch et al., U.S. Pat. No. 4,408,495. There, vibrational movement of the machine to which the fiber optic waveguide is linked causes the change in the waveguide's curvature.
The teachings of Couch and others recognize that use of fiber optic waveguides as sensor elements has several distinct advantages. Primarily, fiber optic sensors are immune to electromagnetic interference while not introducing such interference in and around the object being sensed. Since they are insulators, fiber optic sensors reduce the electrical hazard associated with sensing in high voltage environments. They also are light in weight and offer greater information carrying capacity than most electrical devices. Furthermore, compared to many electrical devices, fiber optic sensors of the type herein discussed are relatively inexpensive to construct and maintain.